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Despite his worries, Mr Blair - who was previously keen on landing himself top EU role - insisted Britain “will vote to stay” in the EU at the in/out referendum on June 23.

The ex-premier also claimed it was the UK’s destiny to “lead in Europe” even though he recognised there is fervent support for Brexit.

Mr Blair’s demand for pro-EU supporters to show more enthusiasm comes after widespread criticism of the so-called ‘Project Fear’ scaremongering being used by David Cameron and other Remain campaigners.

Labour leader Jeremy Corbyn has also been accused of being conspicuously absent from the campaign trail, despite his party’s pro-EU position.

Mr Blair told BBC Radio 4’s Today programme: “The question is, what is in the interests of the people?

"And I would like to see the pro-European side get out there with a bit of passion and vigour and determination and stand up for what we believe.

"And what we believe not just as a matter of economic realism, but as a matter of political idealism."

You can't have a referendum like this, especially with the fervour on the anti side, and not be concerned.

He warned that Brexit would cause economic instability and "damage fundamentally" the interests of the British people, as he conceded he is fretting about this summer’s referendum.

He said: "I think in the end this referendum will be won for those that see Britain remaining part of the European Union.

"But you can't have a referendum like this, especially with the fervour on the anti side, and not be concerned.

"So I am concerned, but I believe in the end we will vote to stay."

With the long-awaited Chilcot report into Mr Blair’s controversial invasion of Iraq still yet to be published, the former prime minister remains a divisive figure in Britain.

And he signalled his unpopularity would lead him to take a back seat during the EU referendum campaign.

He said: "I don't know whether it is the right time for me on the campaign trail - that carries with it negatives as well as positives.

"But I think it is certainly time for people to argue this case for Europe with some passion."

Mr Blair said once the EU referendum was “out of the way” it was Britain’s “destiny” to place itself right at the heart of Europe.

He said: "Britain has got a great opportunity to lead in Europe.

"Britain is a great country, it's a strong country, it's a strong economy - person for person it is in the Premier League of systems."

He added: "Our destiny as a country is to lead in Europe, and we can and we should.

"And once this is out of the way, with some strong leadership in our own country, we will.

"But we have got to realise how fast the world is changing, how important this relationship is, and the massive instability that is going to come for no good purpose if we take the wrong decision."

Mr Blair also hit back at the charge the EU is only popular among the “political elite” while the British public is more sceptical.

He said the UK had "gained enormously" from being part of the EU, adding: “The world that is developing today, it is not to satisfy some elite that we should be part of the biggest political union and biggest commercial market in the world.

"It is precisely to satisfy the interests of the people, those interests that will be damaged fundamentally - not the elites, by the way, supporting this case to get out of Europe - the people are going to be damaged by the instability and the economic difficulty that is going to be consequent from the decision to withdraw."